Some communication systems consist of a base station and a plurality of remote units deployed within a communication field about the base station. Typically, the base station collects information contained in the remote units and may write information into the remote units. The communication field may consist of copper or fiber optic links between the base station and each remote unit. Alternatively, the communication field may be in the form of radio wave transmissions between the base station and the remote units. An example of the latter is a system employing radio frequency identification devices (RFIDs) which are attached to items such as livestock, luggage in airport luggage handling system, personnel badges in a security area, and so on. An interrogator unit (also referred to as a reader unit) broadcasts radio frequency signals to the RFIDs (also referred to as tags), modulated to convey information to the tags. Modulation circuitry in the tags permit transmission of information back to the base station.
In most applications, many tags are present in the interrogation field so that a transmission from the base station typically will be detected by more than one tag. Thus, a request by the base station for information contained in a tag can produce multiple transmissions from many tags, resulting in a garbled response detected at the base station. Conversely, information intended for a tag particular will be picked up by all tags within the interrogation field.
These problems can be avoided by transmitting identifying information so that only the target tag will respond. Oftentimes, however, the base station does not know a priori which tags are deployed within the interrogation field. Moreover, it is often the case that tags enter and leave the interrogation field as a result of the dynamic nature of the environment in which RFID tags are used. Identification schemes have been developed wherein the base station initiates a process by which the deployed tags are identified.
For example, in U.S. Pat. Nos. 5,365,551 and 5,500,650 a system and method are taught wherein a commander station (reader unit) transmits a command to the responder stations (tags), causing each responder to generate a random number which thereafter serves as an arbitration number to identify the responder. The commander station then collects the arbitration number of each responder, thus permitting the commander station to subsequently communicate directly with any one responder station.
In U.S. Pat. No. 5,434,572, a method is disclosed for accessing a desired tag by progressively eliminating the other tags. Each tag is assigned a unique binary ID number having N bit positions. The reader transmits bit commands to the tags. One such command, the BITO command, causes each tag whose current bit position is `0` to respond and each tag whose current bit position is `1` to go into reset. Similarly, a BIT1 command causes each tag whose current bit position is `1` to respond and each tag whose bit position is `0` to go into reset. A response by the tag includes transmitting a return signal and incrementing an internal counter to the next bit position. The reader can therefore gain access to a tag of a given binary ID by issuing an appropriate sequence of BIT0 and BIT1 commands, thus successively putting the undesired tags into reset until only the desired tag remains active.
In U.S. Pat. No. 5,489,908 a system is disclosed for multiple tags wherein each tag has a unique id code. The reader constructs a bit string which is sent to the tags. Each tag compares the bit string against the least significant bits of its id code, and if a mismatch occurs the tag does not respond. Whenever the reader detects multiple responses to a transmission, an additional bit is appended to the bit string and the modified string is transmitted. Additional bits are successively appended until the reader detects that a single response has been received, at which point the code is stored. The entire sequence is repeated until the id codes for all of the tags in the field are recorded.
In U.S. Pat. No. 5,602,538, a multiple tag system is taught wherein the tags transmit their id codes in response to receiving an interrogation command from the reader. The signal from the tag closest to the reader is detected and its id code is stored. The reader then transmits another interrogation command including the received id code. All tags respond by sending their id codes, with the exception of the tag whose id code is included in the transmission. Again, the reader detects the strongest signal and records the id code contained in the signal. The reader transmits yet another interrogation command, this time including the first and the second id codes. Again, all tags respond with their id codes, with the exception of the two tags whose id codes are contained in transmission. The reader detects the strongest signal and stores the id code contained in the signal. This is repeated until all the tags in the interrogation field have been identified.
U.S. Pat. No. 5,294,931 teaches a different approach whereby individual tags are accessed in a multi-tag environment based on each tag's proximity to the reader unit. Each tag is configured to respond to interrogation only when charge voltage generated by the interrogation pulse falls within a fixed voltage range. Thus when the reader emits an interrogation pulse at a given energy, tags close to the reader will be charged at voltage levels above the voltage range and thus will not respond. Similarly, distant tags will be charged at levels below the voltage range and thus will not respond. Only a tag which is located at a proper distance from the reader will be charged to a level that falls within the voltage level and thus will respond to the reader. By varying the energy of the emitted pulse, the reader is capable of interrogating all of the tags in its field.
Each of these prior art approaches require additional circuitry and/or intelligence in the base station and/or the RFID tag to implement an identification protocol. The '931 patent, for example, requires precision components in the tags in order to detect the narrow voltage windows within which operation is permitted.
A need therefore exists for an RFID tag identification method that minimizes the complexity of the design and implementation of the base station and the RFIDs comprising the system. It is desirable to have a method that is applicable to similar communication systems wherein a control unit communicates with two or more remote units.